Glycolic acid vs lactic acid: the two common AHAs compared

Two AHAs, one carbon apart

The FDA names glycolic acid and lactic acid as the most common AHAs in cosmetic products.¹ Both are small molecules: glycolic acid is 2-hydroxyacetic acid, with two carbon atoms, and lactic acid is 2-hydroxypropanoic acid, with three.²³ What makes either one an AHA, and how AHAs differ from salicylic acid, is covered in AHA vs BHA.

As acids they are close. A study reviewed by the EU's scientific committee used glycolic acid as its worst-case AHA, noting that its pK, a measure of acid strength, is comparable to lactic acid's and lower than that of other hydroxy acids.⁴

Side by side

The US Cosmetic Ingredient Review panel's conclusion covers both acids, their common salts and their simple esters. For consumer products it adds a condition: they should be formulated to avoid increasing sun sensitivity, or their directions should include daily sun protection. Salon products must also come with directions for daily sun protection.⁹ The FDA describes the panel as the industry's self-regulatory body for reviewing the safety of cosmetic ingredients.¹

The EU figures are not law. The committee suggested them in 2000 on the precautionary principle, because it lacked the data for a full assessment, and recommended warnings to avoid the eyes and to protect against UV. In 2004 it kept that position because the new data submitted were inadequate.⁴ Neither acid has an entry in the annexes of the Cosmetics Regulation.¹⁰

The sun advice covers both

Most of the evidence on sun sensitivity comes from glycolic acid. In the human studies the FDA reviewed, glycolic acid at concentrations as low as 4%, applied for as little as four days, altered the skin's response to UV, and the change was no longer evident a week after stopping. The FDA nonetheless believes the effects of other AHAs may be the same.⁹

So its recommended sunburn alert applies to any cosmetic with an AHA as an ingredient, lactic acid included, whatever the product claims. The FDA found exfoliation claims on only about half of AHA products, and says it is the presence of the AHA, not the claim, that may increase the possibility of sunburn. AHAs present only as incidental ingredients, at insignificant levels and with no technical effect, are outside the guidance.⁹ The alert's wording is quoted in AHA vs BHA.

Why lactic acid's place on a list means different things

CosIng lists lactic acid for buffering (altering or maintaining a product's pH), as a humectant (retaining moisture in the product during use) and for skin conditioning, and glycolic acid for buffering only. Neither entry lists exfoliating.⁶⁵¹¹ The US panel's report says the same of AHAs in general: they can act as mild exfoliants, but are also used as pH adjusters and skin-conditioning agents.¹²

So one name can be doing different jobs, and its position is the only hint a label gives. High on a list, lactic acid is one of the larger ingredients by weight. Listed after Phenoxyethanol, a preservative capped at 1% in the EU (Annex V, entry 29), it is in practice at about 1% or less, because only ingredients under 1% may be listed out of order.¹⁰ Ingredient list order explains that clue.

On the label

• Glycolic Acid and Lactic Acid are the names to look for. Their sodium salts are separate ingredients, listed as Sodium Glycolate and Sodium Lactate.⁷⁸
• Esters have their own names, such as Methyl Lactate or Cetyl Lactate, and were part of the same US review.⁹
• Note the position, and whether a preservative such as Phenoxyethanol comes before the acid.
• On US packs, look for the sunburn alert. It is recommended, not required.⁹
• Paste the whole list into the ingredient checker to see every acid in it at once.

No label states an AHA's concentration or the product's pH, the two numbers both reviews were written around. A name on the list tells you the acid is there, not how much it does.